Project Summary Autoimmune disorders affect 5-10% of the population and about 80% of these patients are women. Sex chromosomes appear to play an important role in autoimmunity, yet experiments demonstrating causality are missing. Autoimmune disorders like systemic lupus erythematosus (SLE) have no cure, and intervention strategies for correcting genetic abnormalities in lupus hold great promise. Genes from the sex-linked X- chromosome are commonly overexpressed in SLE and are thought to contribute to disease progression. Female mammals, who have 2 X chromosomes (XX), silence one X in a process called X-Chromosome Inactivation (XCI), thereby equalizing X-linked gene expression with males (XY). XCI is initiated and maintained by expression of the long noncoding RNA XIST. It is unknown whether increased X-linked gene expression in SLE results from enhanced transcription from the active X (monoallelic) or from reactivation of the inactive X (biallelic). Remarkably, we recently discovered that the inactive X in nave lymphocytes lacks heterochromatin marks and XIST RNA localization, and upon stimulation, these epigenetic modifications return to the X in less than half of the transcriptionally active cells. This proposal will test the hypothesis that inefficient XIST RNA recruitment to the inactive X impairs the acquisition of heterochromatin marks, thereby increasing the potential for partial X-reactivation and abnormal overexpression of autoimmunity associated genes. We will also determine if biallelic expression of X-linked autoimmunity related genes is increased in pediatric SLE patients and predisposes female mice to develop SLE-like symptoms. Last, we will use our novel X-chromosome silencing system to determine if we can decrease the expression of X-linked genes that contribute to SLE. Our results, the first to define the contribution of XCI to autoimmunity, will provide insight into SLE pathogenesis and identify epigenetic mechanisms as future targets for SLE therapy.